Twin screw rotors for installation in displacement machines for compressible media

ABSTRACT

Displacement machines for compressible media having single-threaded, twin screw rotors with continuously decreasing pitch to achieve an inner compression are known. The demand for compact pumps with rotors as short as possible, and the frequent, process-related demand for quick disassembly of the housing, set limits however for the wrapping angle or respectively for the variation of the end profiles, which, in turn, leads to extreme pitch ratios or to insufficient compression rates. The present invention uses screw rotors with an optimized, non-monotone pitch course and alternative end profile variation with constant diameters. Suction capability is improved with the same space requirement, and compression rates of 3.0 and more are achieved easily with only four wraps. Displacement machines equipped with such twin screw rotors provide optimal results with respect to energy requirement, temperature, construction space and servicing with application possibilities in chemistry and in semiconductor technology.

BACKGROUND OF THE INVENTION

The invention relates to twin screw rotors for installation indisplacement machines for compressible media, in particular pumps, whichrotors are designed single-threaded with varying pitch and are intendedto be in axis-parallel, opposed, outside engagement with wrapping anglesof at least 720°, and to form in a housing an axial sequence of chamberswithout blow-hole connections, the end profile comprising a core arc ofa circle, cycloidal hollow flank, an outer arc of a circle and a furtherflank.

DESCRIPTION OF RELATED ART

The patent DE 87685 shows a machine called a screw wheel capsulemechanism, in which the screw rotors are designed with variable pitch.The machine can be used both as a motor and as a pump. In order toadditionally increase the volume of the working chambers in thedirection of the expanding medium, when operating as a motor, the rotorsare alternatively constructed tapering.

Described in the patent DE 609405 is an air cooling machine withcompressor and expander, which both have screw pairs with variable pitchand thread depth. The enveloping surfaces of the rotors are designedtapering.

Both of the aforementioned machines have the drawback that they requiretapering cylinders, whereby the rotors can be put in and taken out onone side only. This increases the expenditure (of time and effort)during assembly and disassembly of the machines, which is verydisadvantageous in particular during maintenance and repair work.

The patent application EP 0 697 523 concerns a screw-type displacementmachine in which the rotors meshing with one another have unequal screwprofiles, designated as “male” and “female”, so-called S.R.M. profileswith continuous change of the pitch. The end profile is thereby variedin such a way that the angle of the tooth tip, or respectively thelength of outer arc is a monotonically increasing function dependentupon the wrapping angle. Such profiles have the drawback that a goodpartitioning of the axial sequence of working cells is not possibleowing to the remaining blow hole. The vacuum losses caused by the blowhole result in losses in degree of efficiency, so that with such amachine no good inner compression is possible at least at low and mediumrotational speeds.

The published patent application DE 19530662 discloses a screw suctionpump with outer combing screw elements, in which the pitch of the screwelements continuously decreases from their inlet end to their outlet endto cause the compression of the gas to be released. The shape of theteeth of the screw rotor has an epitrochoidal and/or Archimedean curve.This machine has the disadvantage that the achievable inner compressionrate with the shown geometric proportions is mediocre. Moreover thelacking end profile variation sets the already not good compressionratio, and leads to an increased leak rate owing to the reduction in thedepth of the gap between screw outer diameter and housing toward thescrew end.

The published patent application DE 4445958 describes a screw compressorwith opposite-running, rotating outer combing screw elements. Thethreading helices of the screw elements become continuously smaller fromone axial end to the second axial end removed therefrom. Proposed as aprofile is a rectangular or trapezoidal profile. A drawback of thesekinds of geometry for the profile is that they work sufficientlyloss-free only if the thread depth is minimal with respect to thediameter, as is explained in the said publication. Such a machinetherefore has a large constructional volume and a great weight. It is afurther drawback of such profile geometry that extremely high changes inpitch are necessary if a satisfactory inner compression rate is supposedto be achieved. As with the mentioned DE 19530662, the lacking endprofile variation here too fixes this shortcoming, and this leads to ahigh leak rate owing to the reduction of the depth of the gap betweenscrew outer diameter and housing toward the screw end.

Further publications, such as, for example, SE 85331, DE 2434782 and DE2434784, concern inner-axial screw machines with non-constant pitch ofthe screws or varying end profiles. These machines all have the drawbackthat the construction cost is high, and that in each case dynamic sealsare also required suction-side.

Furthermore there are some publications, for example DE 2934065, DE2944714, DE 3332707 and AU 261792, which describe two-shaft compressorswith screw-like rotors. There the rotors, and in some cases the housingtoo, are composed of profile disks of differing thickness and/orcontour, disposed axially behind one another, and thus achieve innercompression. All machines with screw-like rotors have the drawback thattheir degree of efficiency is decreased compared to that of machineswith screw-shaped rotors because detrimental spaces and whirl zonesarise through the stepped construction. Furthermore problems withrespect to shape consistency are to be expected with screw-like rotorssince they heat up in operation.

SUMMARY OF THE INVENTION

Starting with this state of the art, the invention has as its object topropose a twin screw rotor which does not have the above-mentioneddrawbacks.

These objects are attained, according to the invention, in that thepitch is not monotone and is defined as a variable dependent upon thewrapping angle, in that the pitch in a first section increases from thesuction-side screw end, and reaches a maximal value after approximatelyone turn, in that the pitch in a second section, adjacent to the firstsection, decreases and reaches a minimal value at approximately one turnbefore the delivery side screw end, and in that the pitch in a thirdsection, adjacent to the second section, remains substantially constant.

Special embodiments of the invention are described in the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described more closely in the following withreference to the embodiments illustrated in the drawings:

FIG. 1 shows a pair of screw rotors in engagement with one another;

FIG. 2 shows the right-hand rotor;

FIG. 3 shows the left-hand rotor;

FIG. 4 is a frontal section of a screw rotor with varying profile;

FIG. 5 shows the evolution of the reference spiral of a rotor accordingto the invention;

FIG. 6 shows the pitch course of the evolution according to FIG. 5;

FIG. 7 shows the course of the working chamber section for a machinewithout and with profile variation;

FIGS. 8a to 8 c are housing, rotor and working chamber sections of acompressor provided with rotors according to the invention;

FIGS. 9a to 9 p are frontal sections through a rotor pair, whichillustrate the development of the working chamber section according toFIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment example of a pair of screw rotors 1 and 2 inaxis-parallel, outer engagement. FIGS. 2 and 3 show each of the rotors 1and 2 according to FIG. 1 separately. It is clearly discernible in thesefigures that the outer casing and the core are cylindrical, and thus thethread depth is constant over the screw length. In FIG. 3 themeasurements Δw₁ and Δw₃ show that the pitch of the screw varies alongthe axis, that the height h of the screw-shaped outer cylindrical rotorsurface, however, remains thereby constant, whereby leakage lossesbetween rotor and housing inner wall from suction-side screw-end 9toward the delivery side screw-end 10, as arise undesirably with some ofthe initially mentioned state-of-the-art machines, are prevented.Designated by the reference number 7 in this figure is the referencespiral, which will be gone into more closely later on, and 8 is thesecond demarcation spiral of the outer, cylindrical rotor surface.

Shown in FIG. 4 is a rotor in a section at right angles to the rotoraxis. The thus formed end profile has a core arc of a circle 3 with aconstant radius Rb over the entire length of the screw,which—clockwise—after a sector angle γ, changes into a cycloidal hollowflank 4. The geometry of the hollow flank 4 is unchanged over the entirescrew length. Following, with a sharp angle, the hollow flank 4 at pointB₀ is an outer arc 5 with a constant radius Ra over the entire screwlength, which, after a sector angle γ at point C, changes into a furtherflank 6. This comes out finally with a tangential transition into thecore arc of a circle 3. The variation of the end profile contour alongthe screw axis is based on a change of the sector angle γ as well as ona change in the geometry of the further flank 6. The variation of thesector angle γ along the screw axis is preferably spatially determinedthrough an at least close to equidistant course of a reference spiral (7in FIG. 3), formed by the outermost points of the hollow flank B_(α),and the second demarcation spiral (point C) (8 in FIG. 3) of the outer,cylindrical rotor surface.

FIG. 5 shows in a diagram the evolution w of the reference spiralmentioned in connection with FIG. 4 in dependence upon the wrappingangle α. For comparison, a straight line g and corresponding segmentsP₀, 2P₀, etc., of the evolution of a spiral with constant gradient havebeen drawn in. The course of the gradient is clearly discernible if oneconsiders the first derivative, presented in FIG. 6,$w^{\prime}( {= \frac{\delta \quad w}{\delta\alpha}} )$

of the evolution w. This value w′ is the dynamic gradient of theabove-mentioned reference spiral. One sees in this figure that thegradient begins at α=0 with the value $\frac{L_{0}}{2\quad \pi},$

whereby L₀ is a constant corresponding to one of the middle gradientheights. In a first section T₁, w′ increases, and reaches at α=2π themaximal value $\frac{( {1 + A} ) \cdot L_{0}}{2\quad \pi},$

whereby A is the amplitude moderator. In the second section T₂, whichruns from 2π to 6π, the gradient decreases, and reaches at α=6π theminimal value $\frac{( {1 - A} ) \cdot L_{0}}{2\quad \pi},$

which it maintains in the third section T₃ until the end of the screw.

FIG. 7 shows the chamber cross-section F as a function of the wrappingangle α, the curve indicated by an unbroken line F₀ showing the chambercross-section without profile variation and the curve indicated by abroken line F_(m) showing the chamber cross-section with profilevariation.

Shown in FIG. 8a is the cross-section of a housing intended to receivetwin screw rotors according to the invention. FIG. 8b shows a sectionthrough the rotors corresponding to the illustration of FIG. 4, and inFIG. 8c the chamber cross-section F_(o) is shown hatched.

Shown in FIGS. 9a to 9 p is, in frontal section, the development of theworking chamber cross-section in dependence upon the wrapping angle. Thelatter is indicated in angular degrees in the figures.

What is claimed is:
 1. Twin screw rotors for installation indisplacement machines for compressible media, the screw rotors beingsingle-threaded with varying pitch and intended to be in axis-parallel,opposed, outside engagement with wrapping angles of at least 720°, thescrew rotors forming in a housing an axial sequence of chambers withoutblow-hole connections, the twin screw rotors comprising: an end profilethat includes a core arc of a circle, a cycloidal hollow flank, an outerarc of a circle and a further flank; and wherein the pitch of the twinscrew rotors is not monotone and is defined as a variable dependent uponthe wrapping angle, so that the pitch in a first section increases froma suction-side screw end, and reaches a maximal value afterapproximately one turn, the pitch in a second section, adjacent to thefirst section, decreases and reaches a minimal value at approximatelyone turn before a delivery-side screw end, and so that the pitch in athird section, adjacent to the second section, remains substantiallyconstant.
 2. The twin screw rotors according to claim 1, wherein endprofile contours along the screw rotor axis vary in that a sector angleof the core arc of a circle and outer arc of a circle change and thegeometry of the further flank changes, the core circle radius, the outercircle radius and the geometry of the hollow flank being constant, andthe outermost points of the hollow flank forming a reference spiral fordefinition of a pitch course.
 3. The twin screw rotors according toclaim 2, wherein the variation of the sector angle of the core arc of acircle and outer arc of a circle along the screw axis is determined by aspatially at least almost equidistant course of the reference spiral andof a second demarcation spiral of the outer, cylindrical rotor surface.